Unexpected tunnelling volume losses in the Westminster area, London

During construction of the running tunnels for the Jubilee Line Extension, London, large variations in tunnelling volume loss were measured over about a 2 km distance between Waterloo, south of the Thames, and the area north of St James's Park. At the design stage, a value of volume loss of 2% was adopted, which was thought to be conservative. In practice, volume losses of greater than 3% were measured in Westminster and in St James's Park, south of the lake. North of the St James's Park lake volume losses were less than 2%, as expected. Reliable estimates of volume loss are required for the planning and approval of future tunnelling projects in London, and so a detailed investigation was undertaken to establish the reasons for the observed differences. Two primary and interlinked causes for the large volume losses are identified. First, the method of tunnelling and construction control: south of the lake up to 1·9 m of unsupported tunnel heading was often advanced ahead of the shield, and as a result about 50% of the measured volume loss occurred in front of the shield. Second, the geological conditions: the crown of the westbound tunnel lies just below, and the invert of the eastbound tunnel just above, a division of the London Clay that contains numerous sand and silt partings. There was therefore a considerable potential for loosening and softening, particularly with excavation significantly ahead of the shield. Also, there is less clay cover and lower strengths to the south of the lake owing to past erosion (the study area straddles two of the Thames terraces). This case study highlights the importance of taking the engineering geology into account, even in a city such as London where there are decades of experience of tunnelling and the ground conditions are well characterised. The message is equally applicable to many other cities where large subsurface infrastructure projects are planned.